1. Field of the Invention
The present invention relates to iron oxide pellets which are to be reduced in a rotary hearth furnace or the like and to a method of producing the iron oxide pellets. The present invention also relates to reduced iron pellets obtained through reduction of the iron oxide pellets and to a method of producing the reduced iron pellets.
2. Description of the Related Art
The Midrex method is a well-known method of producing reduced iron. In the Midrex method, a reducing gas produced from natural gas is fed through a tuyere into a shaft furnace and allowed to rise therein for reduction of iron ore or iron oxide pellets charged therein, to thereby produce reduced iron. However, since the method requires a supply, as a fuel, of a large amount of high-cost natural gas, the location of a plant utilizing the Midrex method is limited to a region producing natural gas.
In recent years, a certain type of methods for producing reduced iron has become of interest, in which instead of natural gas relatively inexpensive coal can be used as a reducing agent. An example of such a method is disclosed in U.S. Pat. No. 3,443,931. In this prior art technique, a mixture of a powder of iron ore and a carbonaceous material is pelletized, then reduced in a high-temperature atmosphere to thereby produce reduced iron. This method has the following advantages among others: Coal can be used as a reducing agent; a powder of iron ore can be used directly; reduction is performed at a high rate; and the carbon content of a product can be regulated.
However, since carbonaceous material has substantially no effects of binding pellet granules together, the strength of carbonaceous-material-containing iron oxide pellets is low as compared with that of pellets containing no carbonaceous material. If the strength of green pellets before drying is low, the pellets are crushed and pulverized in the handling during the drying process, resulting in a low yield of iron oxide pellets. Also, if the strength of iron oxide pellets after, drying is low, the pellets are crushed and pulverized when fed into a reducing furnace, resulting in a low yield of reduced iron. The pulverization occurring during feeding of the pellets also leads to lowered quality of reduced iron pellets.
Japanese Patent Publication (kokoku) No. 52-26487 discloses a prior art technique directed to improvement of the strength of reduced iron pellets in a reducing process and that of dried iron oxide pellets. In this prior art technique, bentonite (a coagulating agent) is added in an amount of 1 mass % or more to a combination of fine powder of ore material and a carbonaceous reducing agent, and the resultant mixture is kneaded with conditioning water prepared by dissolving a dispersing agent (0.3 mass % or less) in an organic binder such as starch, and granulated while an adequate amount of water is sprayed thereon, to thereby obtain pellets.
This prior art technique enables improvement of the strength of pellets, but has disadvantages as follows:
A first disadvantage will be described. Since bentonite serving as a coagulating agent has a property of swelling to a great extent, a large amount of water must be added during the pelletization step by use of a pelletizer. Addition of water leads to softening and easy deformation of pellets. The deformation hinders the ventilation of drying gas in the drying process so that a long time is required for attaining sufficient dryness. Further, since pellets deformed into a flat shape have low strength, the pellets are susceptible to crushing and pulverization when fed into a reducing furnace. In addition, as the bentonite content increases, the mean grain size of green pellets decreases.
Next will be described a second disadvantage. Since bentonite remains as an impurity in reduced iron pellets, the amount, of slag increases during steelmaking through reduction of reduced iron pellets. This means that the product value of the reduced iron pellets is lowered. In addition, the addition of bentonite increases the cost.